The present invention is generally related to semiconductor devices and more particularly to processes for forming semiconductor devices.
Smaller semiconductor devices are requiring new materials to be used in order to make higher performance semiconductor transistors. Two areas of focus include the use of high-k gate dielectric materials and metal gates. Mid-band gap level materials are being investigated for use as metal electrodes. Materials being investigated for these metal gates include refractory metals such as titanium, tantalum, and tungsten compounds. Tungsten has a problem in that it is typically formed with a reaction of tungsten hexafluoride (WF6) and molecular hydrogen (H2) gas. During the reaction, a significant amount of fluorine is incorporated into the film as it is deposited. As the wafers are further processed, the fluorine leaves the tungsten and can diffuse into the gate dielectric. In the gate dielectric, the fluorine can degrade the quality of the gate dielectric and decreases its dielectric constant. In devices where a high-k gate dielectric is to be used, the fluorine counteracts the effects of the high-k gate dielectric. Further, even if a silicon dioxide gate is used, the effective dielectric constant of the film is reduced and decreases the amount of capacitive coupling between the gate electrode and the underlying semiconductor substrate. Other problems with the use of the WF6 and H2 reaction is that too much fluorine can cause adhesion problems between an oxide layer and a metal layer as well as higher leakage currents for the transistors formed.
The use of some refractory metal nitrides, by themselves, may be insufficient to block the fluorine penetration. For example, if a typical titanium nitride layer is covered by a conventionally formed tungsten layer, fluorine can still penetrate through the titanium nitride during subsequent anneal steps and cause similar problems.
Physical vapor deposition of the metal gate material may not be a good solution. During a physical vapor deposition, typically, a plasma is generated. During the sputtering process the gate dielectric may become damaged or actually remove some of the gate dielectric. Therefore, PVD for metal gates typically is not preferred. A chemical vapor deposition from a carbonyl source (e.g., W(CO)6) may have problems with carbon contamination.